I think you mean this? http://www.gtaforums.com/index.php?showtopic=418025
To do this you must change the vehicles in-game name to 'Banshee' or 'Sultan RS' or any other car which has the exhaust flames by modifying the 'Vehicles.ide' file.
It changes the engine noise and
any visual effects
associated with that engine (Flames, Exhaust fx etc.
) to any car you specify. NOTE:
This also changes the car name that appears when you enter the vehicle, to the one you replace it with.
So if you replace the 'minivan' with 'banshee' engine sound and effects, It's name will appear as banshee when you enter it.
You can change/swap engine sounds and effects with most vehicles, so Heli's with Heli's, bikes with bikes etc.
Refer to thread http://www.gtaforums.com/index.php?showtopic=418025
Hope this helps